Synthesis and properties of mesogenic laterally fluorinated compounds containing benzoxazole unit

Abstract

Series of mesogenic laterally fluorinated compounds, 2-(2′,3′-difluoro-4′-alkoxybiphenyl-4-yl)-benzoxazole derivatives (nB-Fx) bearing different substituents (H, CH₃, Cl, NO₂, coded as nB-FH, nB-FM, nB-FC and nB-FN, respectively) at 5-position, were prepared and characterised. Their phase transition behaviour was investigated by differential scanning calorimetry and polarising optical microscopy. nB-Fx with alkoxy chain lengths of 2 to 10 carbons exhibited enantiotropic mesophases, for which the mesophase ranges were 0°C–58°C and 0°C–71°C on heating and cooling for nB-FH, 41°C–93°C and 66°C–140°C for nB-FM, 44°C–133°C and 87°C–155°C for nB-FC, and 0°C–76°C and 0°C–95°C for nB-FN, respectively. Compared to non-fluorinated analogues, with the exception of nB-FC, fluorinated nB-Fx mainly exhibited nematic mesophase both in heating and cooling, which were attributed to the disruption of the side-to-side intermolecular packing caused by the two ortho-lateral fluoro substituents. For nB-Fx series, nB-FM, nB-FC and nB-FN exhibited a much wider mesophase range than the corresponding nB-FH series, which indicated that the substituent at benzoxazole moiety was helpful in increasing the mesophase stability. With the exception of nB-FN, the nB-Fx series displayed intense photoluminescence emission at 379–383 nm in methylene chloride solution, when it was excited at its absorption maxima.

Keywords:

• laterally fluorinated compounds
• benzoxazole derivative
• mesophase
• photophysical property

Funding

This work was supported by the National Science Foundation Committee of China [grant number 50802058, 51373092]; Specialised Research Fund for the Doctoral Program of Higher Education [grant number 20130202120010]; Program for Changjiang Scholars and Innovative Research Team in University [grant number IRT1070]; Program for Key Science & Technology Innovation Team of Shaanxi Province [grant number 2012KCT-21]; Natural Science Foundation of Shaanxi Province [grant number 2012JM2002]; Key Technologies R&D Program of Shaanxi Province [grant number 2012K08-09]; the Fundamental Research Funds for the Central Universities [grant number GK201302036, GK201302037].